Posted
by
Soulskill
on Friday May 06, 2011 @12:50PM
from the good-sound-walls-make-good-neighbors dept.

disco_tracy writes "Imagine a room where a band is playing. Neighbors can't hear the music, but if someone outside the room is talking, the musicians can hear it. The concept — a kind of one-way mirror for sound — seems imaginary, but two Italian scientists recently pushed this kind of sound manipulating technology closer to reality (abstract)."

I always wondered if it would be possible to build a huge parabolic acoustic "mirror" that could focus the energy of those 1200 watt bass systems back either to the glass or the eardrums of the driver causing aural devastation. How big would this parabola have to be to get the kind of energy that could do some real damage?

You wouldn't want a parabola - to reflect back to the point of origin, you'd want a sphere. It'd be impractical though, due to the impossibility of tracking a moving car. Remember they are at ground zero for every note, so the car is built for it and the driver willing to sacrifice his hearing.

Well, if it's controllable it would be nice having it on car windows, or if it isolates certain frequencies, you could use those frequencies for emergency vehicles so some @$^!#$% people won't block their way.

I don't expect that the scientists in question longed to solve the problem of the band that needs to play loud music but still hear the people outside. They probably did it for the science itself, which seems fascinating.

What's wrong with an ordinary soundproof wall with a microphone on one side and a speaker on the other?

That's like saying why do we need one way mirror when we can just build a wall and put a video camera on one side and a monitor on the other? I'm sure there will be cases when something like this would be useful.

The same reason why a 1 way mirror is better in some applications than a video camera on one side of a wall with a monitor on the other.

On possible use is in security. People can avoid cameras, and small unseen microphones are not directional. Airports, casinos, police stations, and other security heavy areas will certainly have a use.

Imagine the CIA setting up a room for foreign dignitaries. The dignitaries sweep the room for bugs. But unknown to them, the floor of their room is a 1 way sound barrier, with agents sitting below with directional microphones pointed at the ceiling underneath each room.

Imagine a submarine with a section of the vessel being a 1 way sound room where large microphones reside. All other walls are sound deadened except 1 outside wall. More equipment could sit there than any outside array of microphones, listening for enemy ships... but without an outside sonar signature, and without worrying about hearing internal noises.

Even naturalists would love it. Imagine a retreat in a forrest where every outside wall was 1 way, making it sound like you weren't even in a building, but the animals were not disturbed by the sounds you make snapping pictures and talking.

You're assuming the sound would remain directional, which may not be the case. Indeed, I would expect the sound to be projected perpendicular to the surface.... and it will probably not be as general as you expect. It will be a long way (if at all) until it would look like anything other than what it was.

That is done in studios currently, but I think the main downside it requires electricity and someone usually has to push a button to turn it on (in my experience). Not really sure how effective it would be in a studio, though, unless you want a lot of natural reverb. Incidentally, I used to have a room set up with sound absorbing tile for that exact reason, but that all got torn out in a remodel a few years ago (it was in really crappy condition, or in my wife's words, was "fuggly and has to go"). I actuall

I'm not sure this would be of any use in a recording studio. Typically they use monitors, and it's not because of the wall. The reason for the sound proofing is because the audio engineer is more interested in what they've got on tape than what's actually being produced. Because ultimately that's what they're going to have available when they go to master the CDs, not the actual sound that's being produced.

You really wouldn't want to turn it around either, because you don't want to have to find completely s

I am not sure quality and volume are necessarily equivalent. Actually, I can attest that I have heard quite a few very loud, yet horrendous bands on any given Monday night at the Rat. (if you recognize the club, you're probably old too...)

Not only that, but they've got the use case backwards. The proper way to set it up is so that everyone else hears the band, but the band can't hear any of the complaints from the neighbors banging on the walls and yelling at them.

Although the acoustic technology remains theoretical, the researchers think that it can be built. Lepri cited optics, saying that nonlinear photonics is a well-developed field now.

Couldn't i just sound proof a room, put a few microphones outside the room send the signal in to the room. Doing it passively is cool but there is no way it would be cheaper then some soundboard a microphone and a speaker.

There's that, but there's also the question of why that would be desirable anyways. I assumed that meant between sets, but even that doesn't seem particularly useful. If you've genuinely blocked the sound from penetrating in that direction the people on the other side of the wall shouldn't have anything relevant to say.

The article describes the proposed theoretical mechanism as a "wave diode" - I wonder if the same principle is applicable to light? I don't know if "light diodes" already exist in some sense, but that seems like a possibly useful component for optical computing...?

There is such a thing for light, an optical (or Faraday) isolator [wikipedia.org], which uses the Faraday effect. It relies on the fact that light is a transverse wave, not longitudinal, so it wouldn't work for sound. Note that because of the large magnetic field required, most optical isolators only have an aperture of a few millimeters (or less), so it would be impractical to build a "one-way" mirror of appreciable size out of it.

No... a one-way mirror doesn't allow light to pass in only one direction. It passes, and reflects, light equally in either direction. The trick is to make it dark enough on one side that you can't see your own reflection, and bright enough on the other side that they can't see you because their reflection is too bright.

Ever since I was a kid I've dreamed of having a hot/cold pad. It only lets heat go through it one way, cold side painted blue, hot side painted red. Put it around your soda or beer can blue side in, it cools off your drink, remove when it's just right. Put it around your coffee or soup red side in, it heats it up.

It would probably need a special holding case otherwise your backpack would catch on fire while carrying it.:-)

No, fold it in half with the hot sides facing out. That way there's a limited amount of heat trapped inside it for it to move out; when the middle reaches absolute zero it simply can't continue moving heat across (actually, it'd undoubtedly be an asymptotic approach, but that's beside the point).

What you suggested would cause the inside to get hotter and hotter, making your backpack colder in the process. And even whatever magical material you used to make the hotpad has to have a melting point somewhere...

he wants a passive device. What he's talking about is a thermal diode. thermal and acoustic diodes would be very similar (audible vs phonon frequency matching being the primary issue). if they didn't violate the 2nd law, that is.

This isn't really like a one-way mirror. With a one-way mirror light travels through it and reflects from it the same in both directions. It's just that the amount of light reflecting from the bright side is much much greater than the 'signal' which comes through from the dark side.

Unfortunately for the neighbors, the band is more likely to install the walls the other way around. Making a room that can be monitored from the outside (by a sound engineer/producer) while rejecting outside sounds would be ideal for a recording studio.

You wouldn't; the pickups would be inside the sound chamber, not outside. That's the whole point of rejecting outside sound from the space, get a clean recording of what happens inside.

That being said, though, I think the right question for a skeptic would be "wouldn't a completely soundproof room and earphones for the producers/engineers be a better solution than this?", to which the answer is, "yeah, maybe". After all, that's what we're doing now in good studios, and the job's getting done. I'm sure if

A material that allows evergy to only pass one way seems like it would defy some sort of law of physics. Imagine a cube made of this stuff that only lets sound in... Would the energy inside the cube continue to build until it erupted in a sonic boom?

Almost all materials have differing levels of transmission and reflection of waves. It has to do with impedance difference at the interface.

The trick here is there are two interfaces. Air-to-wall on either side. What they've done is to make the impedance differences dfferent on the two sides (low-to-high on one side and balanced on the other side, I would guess), and make it work for a broad spectrum of frequencies.

The question is how much attentuation you get in the transmissive direction. If they're c

Make a small cube with each wall made of this material so that sound can enter the cube but it can't get out. Next, let it fill up with sound. Take it to the airport, construction sites, concerts, etc...

The reason it won't work is stated a bit above: The soundwaves change to heat (slowly, but they do) in the air and at each bounce. The box would start heating up as you "fill" it and this heat would leak away.

With material like that loudspeakers could be build that won't reflect the sound inside the box back outside through the cone.
One layer of one-way material on the inside, then one sound absorbing layer on the outside.

That's active noise cancellation. This is a proposed mechanism that uses materials with nonlinear acoustic properties to accomplish a similar effect passively. Both methods might be suitable for different situations.

That's not how that works. If you've got the obnoxious music inside the car, there is no way that you could direct this technology where you'd be able to hear the motorcycle. You could however direct it so that they could hear you're obnoxious music, but you couldn't hear them.